Wilms' tumor gene 1 (WT1) encodes a zinc finger transcription factor involved in cell proliferation and differentiation. It is highly expressed in a wide variety of malignancies including several types of hematological malignancies and various solid tumors. In contrast, normal tissue expression of WT1 in adults is restricted to gonads, uterus, kidney, mesothelium and CD34+ progenitor cells in various types of tissues. WT1 was originally proposed as a tumor suppressor gene. However, more recent evidence points to oncogenic functions of this transcription factor; Wt-1 negatively affects differentiation and promotes proliferation of progenitor cells. Furthermore, overexpressed WT1 is immunogenic; WT1 specific T cells as well as IgG anti-WT1 antibodies have been observed in cancer patients. Thus, WT-1 is a promising candidate for the development of cancer vaccines.
Human clinical trials with WT1 vaccines based on HLA (human leukocyte antigen)-restricted WT1 peptide fragments have been reported. Osada et al., Clin Cancer Res 2009; 15:2789-2796, discloses a WT1-encoding Adenovirus Vaccine.
To the inventor's knowledge, no live bacterial cancer vaccine targeting WT1 has been reported. Furthermore, no oral cancer vaccine targeting WT1 has been described.
Attenuated derivatives of Salmonella enterica are attractive vehicles for the delivery of heterologous antigens to the mammalian immune system, since S. enterica strains can potentially be delivered via mucosal routes of immunization, i.e. orally or nasally, which offers advantages of simplicity and safety compared to parenteral administration. Furthermore, Salmonella strains elicit strong humoral and cellular immune responses at the level of both systemic and mucosal compartments. Batch preparation costs are relatively low and formulations of live bacterial vaccines are highly stable. Attenuation can be accomplished by deletion of various genes, including virulence, regulatory, and metabolic genes.
Several Salmonella typhimurium strains attenuated by am mutations have been shown to be safe and effective delivery vehicles for heterologous antigens in animal models.
Approaches of delivering DNA constructs encoding antigens, in particular VEGF receptor proteins, via live attenuated Salmonella typhimurium strains into mouse target cells are described in WO 03/073995. Niethammer et al., (Nature Medicine 2002, 8(12), 1369) demonstrated that the attenuated S. typhimurium aroA strain SL7207 harboring an expression vector encoding the murine vascular endothelial growth factor receptor 2 (VEGFR-2 or FLK-1), which is essential for tumor angiogenesis, is functional as a cancer vaccine.
There is however only one attenuated Salmonella enterica serovar strain, namely Salmonella enterica serovar typhi Ty21a (short: S. typhi Ty21a), which has been accepted for use in humans and is distributed under the trade name of Vivotif® (Berna Biotech Ltd., a Crucell Company, Switzerland; marketing authorization number PL 15747/0001 dated 16 Dec. 1996).
This well-tolerated, live oral vaccine against typhoid fever was derived by chemical mutagenesis of the wild-type virulent bacterial isolate S. typhi Ty2 and harbors a loss-of-function mutation in the galE gene, as well as other less defined mutations. It has been licensed as typhoid vaccine in many countries after it was shown to be efficacious and safe in field trials.
WT1 is a promising tumor antigen for the development of cancer vaccines. Major limitations of previously available WT1 peptide vaccines are HLA-restriction and parenteral administration. The great need for improved cancer therapy approaches based on targeting WT1 has not been met so far.